The present invention relates in particular to insulating undersea pipes installed in oil fields at very great depths, and also to pipes suspended between the sea bottom and a surface vessel anchored over such an oil field.
The invention can also be applied in the field of the aviation and space industries where the concept of weight is of extreme importance.
In most industrial fields, it is desired to have insulation systems that provide good performance in order to ensure that fluids conveyed in pipes are maintained at constant temperature so that transfers between pieces of equipment can be undertaken over long distances, e.g. reaching several hundreds of meters or even several kilometers. Such distances are commonplace in plant such as oil refineries, liquefied natural gas installations (−165° C.), and off-shore oil fields which extend over several tens of kilometers. Such oil fields are being developed in ever-increasing depths of water, and such depths can exceed 3000 meters (m).
Numerous systems have been developed to reach a high level of thermal performance, and specific versions have been developed for being more suitable for use in great depths, i.e. for withstanding pressure at the sea bottom. Water pressure is substantially equal to 1 bar for a depth of 10 m, so the pressure that the pipe needs to withstand is then about 100 bars for a depth of 1000 m and about 300 bars for 3000 m.
The technologies presenting the greatest performance that have been developed for achieving this object are so-called “pipe in pipe” (PIP) technologies in which an inner pipe conveys the fluid and an outer pipe coaxial about the inner pipe is in contact with the surrounding medium, i.e. water. The annular gap between the two pipes is filled with an insulating material, or else is evacuated of all gas.
When using an insulating material of the polyurethane foam type under a gaseous atmosphere, the thicknesses required are generally considerable, e.g. 5 centimeters (cm) or 10 cm, which makes it necessary to use an outer pipe of large diameter. However, the outer pipe must withstand the pressure at the sea bottom without imploding, which requires the steel to be of extra thickness in order to withstand the pressure, and of extra area because of the increase in diameter. Furthermore, the annular gap filled with foam creates additional buoyancy that often needs to be compensated by increasing the weight of the pipe, and this is often done by increasing the thickness of the outer pipe. Thus, very often, the dimensioning of the outer pipe is governed by the requirements for the PIP assembly to be non-buoyant, rather than by the need for it to withstand sea bottom pressure.
In order to mitigate that drawback, technologies have been developed that provide much greater performance, in which it is desired to minimize the size of the annular gap between the pipes, in particular with a vacuum being established in the interstitial zone. The vacuum as created in this way then constitutes an excellent insulator, and an annular space of 5 millimeters (mm) to 10 mm then suffices. A PIP is then comparable in terms of performance to a “thermos flask” made of glass and of the kind in everyday use. In order to improve insulation, radiation is also limited by wrapping the outer portion of the inner pipe in a reflective film, generally constituted by a film of aluminum, possibly in association with thermoplastic materials.
Pipes are then prefabricated in unit lengths of 6 m to 12 m, and then they are laid after being assembled to one another in order to constitute a continuous connection. In the event of one or more segments of PIP being damaged, thermal bridges arise which, so long as they are limited in number and occupy small spots only, do not have significant repercussions on the behavior of the pipe which generally continues to perform its function over time.
Pipes made in this way using PIP technology based on a high vacuum nevertheless present the following drawbacks. The fluid coming from the offshore well is a mixture of crude oil, water, gas, and a variety of aggressive compounds such as CO2 and H2S. The inner pipe, which is generally made of carbon steel, is subjected to corrosion phenomena due to the fluid and having the consequence of producing atomic hydrogen H+, which then tends to migrate in atomic form through the iron and carbon matrix of the steel so as to recombine on the outer wall of the inner pipe, i.e. in the annular gap between the two coaxial pipes, where it forms hydrogen gas of formula H2. The vacuum created in this annular gap then degrades as the pressure of hydrogen increases, and hydrogen is an excellent conductor of heat, which goes against the intended purpose.
Various means have been developed to mitigate that drawback. One known means consists in introducing in the annular gap a compound that is known as a “getter”, having the function of absorbing said gaseous hydrogen in order to keep the hydrogen partial pressure as low as possible, and thus maintain a desirable level of vacuum throughout the lifetime of the oil field, which can exceed 20 years. The getter is inserted during fabrication of the PIP and prior to final sealing, after evacuating.
Proposals have also been made for the inner pipe at least to be made of corrosion-resistant steel, such as stainless steel or duplex steel, however pipes are then extremely expensive and generally do not constitute a solution that is acceptable, economically speaking.
It is also commonplace to cover the inside of the pipe in a protective film, e.g. of the epoxy type, having the function of avoiding contact between the aggressive agents and the carbon steel. Nevertheless, where pipe segments are end-joined together on site, the welding process locally destroys said protection and creates zones that are subject to corrosion. In addition, in the event of the covering being damaged by cleaning tools being passed along the pipe or indeed by abrasion due to sand being conveyed together with the effluent, the surface runs the risk of degrading over time and the phenomenon of migration of atomic hydrogen amplifies over time.
Also known is the technology that has been developed in particular in the field of household appliances that is based on making flat panels out of a porous insulating material confined in a case constituted by a film, and generally heat-sealed at its ends, with a high vacuum being established inside the case prior to final sealing, i.e. a vacuum of the order of 1 millibar or less. The film is generally a multilayer laminate including a barrier that is generally constituted by a continuous aluminum film associated by adhesive or bonding with thermoplastic films, which films serve to allow the assembly to be heat-sealed. Such films are commonly used in the food industry for conserving produce such as milk, fruit juice, and coffee in a neutral atmosphere or in a vacuum.
That of insulation provides good performance since it limits heat transfer by conduction, convection, and radiation and serves to create reflective screens having the function of reflecting energetic radiation. Conduction is limited by depleting the molecules present or by trapping them within a cavity structure inside the porous insulating material, which material is preferably nano-porous. Such porous and nano-porous insulating materials are well known and are described below.
WO 01/38779 in the name of the Applicant describes an insulating tubular complex suitable for being placed around a pipe, and more particularly in the gap between two coaxial pipes comprising an inner pipe and an outer pipe, preferably undersea pipes for use in great depths, the complex being characterized in that it comprises:                a flexible case, and an insulating material confined in a vacuum between the inner wall and the outer wall of said case, said insulating material being preformed in the shape of a rigid tube;        each of said walls being constituted by a flexible multilayer film comprising at least one very thin metal film acting as a molecular sealing barrier, associated with thermoplastic films providing the case with strength and the ability to be heat-sealed.        
In practice, the metal film is as thin as possible in order to avoid thermal bridges at the longitudinal ends of the case, i.e. the film is less than 10 micrometers (μm) thick.
Said nano-porous insulating material is in the form of a tube.
The opposite ends in the longitudinal direction XX′ of said pipe and of said tubular complexes are each constituted by the respective edges of said inner and outer walls of said tubular complex, which edges are sealed together, preferably by adhesive or by heat-sealing.
That configuration is particularly advantageous, since the tubular configuration does not present any discontinuity in the circular radial direction, and the only discontinuities in the insulation are situated at each of the longitudinal ends of said tubular case.
In that patent WO 01/38779 there is described more particularly the manner in which the cylindrical cases are prepared, and in particular the manner in which assembly and welding is performed at the ends of the inner and outer walls by creating folds for the purpose of accommodating the differences in the developed sizes of the inner wall and the outer wall of the case.
In a first embodiment, folds are created in the end edges of the outer wall. In a second embodiment, it is the inner wall that has folds in its central zone in contact with said pipe between the opposite ends of the complex in the longitudinal direction. These folds also serve to deal with the reduction in the diameter of the cylindrical outer wall when the case is evacuated, without any risk of the case becoming leaky. The folds do not present any drawback, since in a case made up of a multilayer film of flexible thermoplastic material and given the flexibility of said case, the folds naturally flatten down one on another without leading to significant extra thickness or even to a zone that is unstable in shape, and the case remains practically circular in overall shape.
The insulating tubular complexes described in WO 01/38779 nevertheless suffer from two main drawbacks:                they must necessarily be prepared in tubular form, which represents a relatively large amount of bulk and leads to difficulties in transportation if they are not fabricated on their site of use, since they are extremely fragile, and in the event of an impact or being punctured merely by a hole or a pin, the tubular complex ceases to be effective;        secondly, the properties of the multilayer thermoplastic films used do not guarantee that a vacuum can be maintained over a lifetime in excess of 20 years, because gas migrates through the multilayer case and through the seals between the thermoplastic materials, even if it does so at a very low rate; and        finally, the ability of multilayer thermoplastic films to withstand high temperatures is very limited, and they cannot be used at temperatures in the range 150° C. to 200° C. as are encountered in certain oil fields.        
Also known are plane panels confined in cases made of metal strips, made of steel, in particular stainless steel, or aluminum, and welded in leaktight manner over their entire periphery, thus presenting a lifetime that is much longer than that of plane panels having thermoplastic cases of the above-described type, since the migration of gas molecules through weld zones is practically zero when the case is made of metal, thus making it possible to achieve lifetimes in excess of 50 years. Patent EP-0 857 833 describes a method of fabricating such a panel in a plane rectangular configuration.
When using plane insulating panels made from metal strips of the kind described in patent EP-0 857 833, it is practically impossible to bend such a panel in order to obtain a circular shape, because of the rigidity of said strip, and also because of the fragility of the wall constituting its edge, since it is constituted by a strip of very small thickness (a few hundredths of a millimeter), in order to minimize thermal bridges between the outer face and the inner face of the evacuated insulating complex.
Similarly, when it is desired to replace the thermoplastic multilayer cases of the insulating tubular complexes of WO 01/38779 with metal strip, even if it is very thin, the local rigidity of said strip remains high so that during evacuation the outer case collapses and presses against the insulating material, leading to buckling similar to that which occurs in a thin steel or aluminum beer can when it is compressed radially by hand. This leads to localized bumps being formed, giving rise not only to points of weakness, but also, and randomly, to a shape that is locally non-circular, presenting more or less sharp angles, which shape may be inscribed in a circle of diameter greater than the nominal diameter of the outer case at rest, thus running the risk locally of interfering with the inside face of the outer pipe of the PIP.
Thus, the problem posed is to provide an insulating complex comprising a nano-porous insulating material confined under a vacuum in a case suitable for being put into place in tubular shape around a pipe, and capable of being used in a wide variety of applications and more particularly for insulating undersea pipes immersed at great depths, and thus satisfying the conditions that are to be encountered when developing oil fields in deep water.
More particularly, the problem posed is that of providing an insulating complex presenting properties and performance that are improved compared with those of the prior art, in particular in terms of mechanical strength, ability to withstand high temperatures, which can reach 150° C. to 200° C. or even more, and a lifetime for the thermal insulation properties that is of the order of 50 years or more, capable of being certified or of satisfying administrative regulations.
Another object of the present invention is to provide an insulating complex capable of being prepared in substantially plane form and suitable for being evacuated, and subsequently bent so as to obtain a circular tubular shape without leading to damaging deformation, whether in terms of the case leaking, or in terms of the pipe in which the case is inserted being damaged, in particular due to said circular shape being deformed by kinking or by ovalization.